This invention relates generally to control of thermal printers and more particularly to methods and apparatus for controlling the energization of a thermal print head in a thermal printer based upon the historical temperature driving conditions at the thermal print head.
A variety of methods have been utilized relative to present day thermal printers to prevent a deterioration in printing quality caused by the accumulation of heat at thermal print head due to continuous thermal operation of the thermal print head. Among the methods employed is a method of holding in memory printing data for each dot to be printed and determining the current flow time to be applied to the thermal print head heating elements based upon the spatial sequence of dots to be printed, such as, disclosed in Japanese patent application No. 55-48631, and a method of changing the current flow time of the pulse to thermal print head heating elements based upon the printing activity of previous print data, such as, disclosed in Japanese patent application No. 57-18507. These forgoing methods may be generally referred to as historical control methods since they contemplate the variance of the pulse width of the drive pulses to be applied to thermal print head heating elements based on previous historical drive data.
In addition, historical thermal print head temperature data methods have been developed that control the applied energization to the thermal print head printing elements based on temperature conditions sensed at the thermal print head. Japanese patent Nos. 61-130063 and 59-7068 are examples of applied thermal print head energy control circuits that measure the baseplate material temperature of the thermal print head employing, in combination, a thermistor and A/D converter and, in turn, calculating means determines the amount of increase or decrease in the applied energy to each heating element of the thermal print head based upon the determined head temperature. In all of these types of thermal printer head temperature compensating and adjustment methods, complicated calculations are utilized in connection with the detected output values of the A/D converter to determine values, such as, the applied energy pulse width and the applied voltage values to the current drivers for operation of the thermal print head heating elements.
In addition, the general method with these historical control method has been sending data sequentially to the thermal head drive IC while generally processing data by means of a CPU. In employing such a method, even if an attempt was made to operate the thermal printer at a high operating speed, the processing of data is not sufficient fast to keep up with the printing operation, which has been a detriment toward the ability to increase the speed of thermal printers.
Reference is made to FIG. 14 which illustrates a conventional thermistor temperature detection circuit comprising the linearized circuits of a thermistor and the A/D converter employed in historical thermal print head temperature data methods. In general, divider circuit 125 comprises resistor 121 connected in parallel with thermistor 120 and resistor 122 is connected in series to thermistor 120 and to the power supply to form a linearized circuit. The voltage potential V.sub.p of voltage divider point 123 of voltage divider circuit 125 is the detected input 115 to A/D converter 110. A/D converter 110 provides an output representing this electric potential in binary code and a CPU connected to A/D converter 110 will read this code and perform arithmetic processing to determine adjustments to be made to the level of energization of thermal print head heating elements. A/D converter 110 is connected via line 112 to the positive (+) terminal of the power supply 112 and via line 114 to the negative (-) terminal of the power supply. Input line 113 to A/D converter 110 is for the detection range setting, and the same power supply is connected as input as in the case of line 112.
FIG. 15 is a graph illustrating the relationship between the electric potential, V.sub.p, of voltage divider point 123 of the circuit of FIG. 14 and the temperature of thermistor 120. In this example, at 25.degree. C., thermistor 120 is R.sub.th =50 kilohms, resistor 122 is R.sub.1 =60 kilohms and resistor 121 is R.sub.2 =500 kilohms. The electric potential, V.sub.p, at point 123 will vary with changes in resistance of thermistor 120 relative to the fixed resistances of resistors R.sub.1 and R.sub.2. However, as is clear from characteristic curve 131 in FIG. 15, the output electrical potential, V.sub.p, reaches saturation as the temperature of thermistor 120 increases since curve 131 increasing flattens out with higher temperature. In addition, because a large voltage range cannot be obtained in the thermistor operating temperature range of 0.degree. C. and 60.degree. C., the electric potential per one unit step increases monotonically and, as a result, the range of the A/D converter detection potential monotonically increases so that detection accuracy is limited at higher operating temperatures. In particular, when the temperature increases due to heat accumulation at the thermal print head, heat control relative the thermal printer heating elements must to take place at 40.degree. C. or higher in order to increase printing resolution and allow for accurate detection. As a result, it was necessary to employ high performance, expensive A/D converters that could provide a large selection of possible binary data values.
More recently is the print controlling apparatus for a thermal printer similar to the type disclosed herein set forth in U.S. Pat. No. 4,912,485, issued Mar. 27, 1990, in the name of the assignee of this application, and is incorporated herein by reference thereto.
It is an object of this invention to eliminate the foregoing problems by providing a thermal printer drive control apparatus that functions at high speed, provides good print quality and has stabilized printing density.
It is another object of this invention to provide simple, low cost A/D converter for the thermal printer drive control apparatus to detect the temperature of the thermal print head and/or the ambient temperature of the thermal print head and control operation at the thermal print head compensate for temperature changes or fluctuations at the thermal print head.
It is a further object of this invention to offer an inexpensive and highly reliable thermal head temperature detection method in which temperature detection can take place accurately by means of improving the thermistor temperature detection circuit without the need for employing a high performance A/D converter, which converter may be a separate component or integrated as part of the CPU, thereby providing for effective cost savings in the design of a thermal printer.